Image Accessibility, Part I: Beyond alt Attributes


The alt attribute is the HTML feature most often associated with accessibility. It’s been around since the HTML 2.0 specification (RFC 1866, finalized in 1995), which noted that “user agents may process the value of the ALT attribute as an alternative to processing the image resource indicated by the SRC attribute.” In plain English, alt attributes were meant as fallback content for web browsers unable to display images, which not all early web browsers could. The World Wide Web Consortium formalized alt as an accessibility feature four years later, in May 1999’s Web Content Accessibility Guidelines 1.0 specification.

The attribute has endured in its association with accessibility, being well known even among people with an otherwise limited knowledge of HTML. Get into a group discussion about how to best achieve accessibility on the web and people will excitedly shout “Alt text!” or “Alt tags!”—although strictly speaking, alt is an attribute on the image tag, <img>. In the extreme, alt attributes are mistaken for image accessibility itself: Include alt attributes, and your images are accessible. Exclude them, and they’re not.

Over a series of two posts, I’m going to present two complementary approaches to delivering accessible images on the Web, based on recent advancements in the HTML5 specification. In this post, I’ll cover methods attending to the longstanding accessibility problem that alt attributes only partly solve: presenting images with text-based descriptions and enhancements that serve all users. The next post will focus on device- and layout-sensitive image delivery now native to HTML in order to address the overlooked problem of the accessibility of images themselves.

Three Limitations to alt Attributes

You have to admire the alt attribute: for a simple language feature, it’s unrivaled in encouraging people to consider accessibility when they write and design for the web. If you’re new to HTML, here is an image tag presented with an alt attribute:

<img src="flowers.jpg" alt="Photo of beautiful flowers." />

In the terminology of HTML, alt and src (source) are attributes. The quoted text flowers.jpg and Photo of beautiful flowers are values, sometimes referred to as attribute values. And img of course is an HTML element, enclosed in angle brackets to make a tag. I prefer XML-style syntax, so I self-close image tags, <img />, but in HTML5 <img>is also acceptable.

To understand why alt attributes alone are insufficient for accessible image presentation, consider their three limitations:

  1. Content: HTML limits attribute values to plain text, such as Photo of beautiful flowers in the example above. None of the semantic tags that make for structurally expressive and accessible content elsewhere in HTML can be used inside of an attribute, alt or otherwise. You can’t, for example, use within an attribute value an <em> tag for providing emphasis on a portion of your alt attribute’s text, such as Photo of <em>beautiful</em> flowers for exceptionally beautiful flowers, nor can you write the <a> (anchor) tag to link to a fuller description of the image from within the alt text, as this example shows.
  2. Discoverability: an alt attribute’s value is not palpable content, the way the text is between opening and closing HTML tags such as paragraphs or headings: <h1>This is Palpable Content</h1>. Ironically, that means the alt attribute’s content is inaccessible to many people. Unless someone is using an assistive device, like a screen reader, or viewing the page under error conditions, like when an image fails to load, the presence of alt text is impossible to detect without examining the source code. The contents of an alt attribute will not aid, for example, someone who can read text on screen if the text is scaled large enough but who does not have the fidelity of vision to discern the details of a photograph, chart, map, illustration, or other visual material. Certain browsers might render the contents of the alt attribute when an image is hovered over with a mouse pointer, but that’s no help to people using touchscreen devices, which are incapable of detecting the hover state of someone’s finger.
  3. Length: certain browsers render only the first 100 or so characters of an alt attribute, particularly in tool-tip text shown to users upon hovering mouse pointers over images. In testing on the most recent version of Safari on iOS and OS X (9.1), I found that alt text will not be displayed in place of a broken image if the text runs longer than one line within the broken image’s dimensions as specified in HTML or CSS, as this example shows if accessed using Safari. Keeping alt text accessible means keeping it short, with short being an arbitrary descriptor even in modern browsers like Safari. Of course, the shorter alt text is, the less descriptive it can be.

Prior to HTML5, the <img> element had a long-description attribute (longdesc), which posed even more discoverability problems than alt, as not even an error condition revealed its presence. It was also frequently misused: HTML authors would stuff even more plain-text descriptive content into the longdesc attribute, instead of a URL pointing to the location of the descriptive content, as called for by the specification. For those reasons, longdesc is obsolete in HTML5, which now directs HTML authors simply to provide a link to any descriptive content via a vanilla anchor tag (<a>).

Despite their limitations, Yes, you should write alt attributes in your <img> tags. But no, you must not stop there, if your aim is an accessible presentation of images on the Web. The richest expressions of web accessibility are those that inclusively enrich the experience of a page or site for all users, regardless of ability.

When I write alt attributes, I always begin with a descriptor of the visual material being presented: Photo of…, Map of…, Chart illustrating… followed by the shortest possible description. Any content beyond that is more accessibly delivered to all users, using semantic elements that I’ll describe next.

HTML Structure Around Images

Images must somehow be structurally presented alongside other HTML content, such as the running text of an article. It’s not uncommon to see markup that looks something like this:

<div class="photo">
  <img src="hank.jpg" alt="Photo of Hank the dog." />

That pattern provides a styling hook in CSS ( or just .photo) for adding borders and other design elements to the image, and it also lends itself to including a caption of some sort, probably as a paragraph element:

<div class="photo">
  <img src="hank.jpg" alt="Photo of Hank the dog." />
    Hank enjoying himself in the flower garden on a hot day.

However, the accessibility of that markup is limited because <div> is a generic, non-semantic element. In the frank words of the HTML5 specification, “the div element has no special meaning at all” and therefore is “an element of last resort.” Adding photo in a class attribute will make sense to humans reading the source code, but it has no meaning to browsers or assistive devices. Neither is there any semantic indication that the paragraph inside of the <div> is a caption. The caption’s proximity in the source code is not enough to make its contents and purpose fully accessible to all.

HTML5 introduced a number of new semantic elements as better alternatives to generics like <div> and <span>. The new element for marking up images and other media is <figure>, which is “used to annotate illustrations, diagrams, photos, code listings, etc.” <figure> should also include its semantic child element called <figcaption> (figure caption). Here is the example above rewritten using the new semantic elements in HTML5:

<figure class="photo">
  <img src="hank.jpg" alt="Photo of Hank the dog." />
    Hank enjoying himself in the flower garden on a hot day.

Unlike <div>, <figure> has a clear and well defined meaning in modern browsers and assistive devices. It’s semantic. The <figcaption> inside the <figure> element reflects a semantic association that informs browsers and assistive devices that the caption is specifically for the media presented inside of the <figure> element: the <img> tag, in this case. And unlike content tucked inside the alt attribute, the palpable content tagged by <figcaption> is presented to and therefore discoverable by all users.

ARIA Attributes

Accessible Rich Internet Applications (ARIA) attributes enhance HTML5’s native structural associations, such as those between <figure> and <figcaption>. ARIA attributes are specifically designed to reduce ambiguity of the function or purpose of a web page’s markup and interactive components. ARIA is in active development by the W3C’s Web Accessibility Initiative, the umbrella organization responsible for the Web Content Accessibility Guidelines mentioned above, currently in version 2.0. There is also a specification in development for ARIA in HTML, which when complete will clarify the use of ARIA alongside HTML’s native semantics. But there is no need to wait for that specification. ARIA can be used in HTML immediately.

The first ARIA attribute to enhance the example code I’ve written so far is aria-labelledby. Note the British English spelling of labelled, with two Ls. The value of that attribute must be a unique identifier assigned using the id attribute on some other HTML tag marking content that functions as a label for the image. In this example, the caption itself is an obvious label, so I’ve added id="garden-photo-caption" to <figcaption> and set garden-photo-caption as the value of aria-labelledby on the <figure> element:

<figure class="photo" aria-labelledby="garden-photo-caption">
  <img src="hank.jpg" alt="Photo of Hank the dog." />
  <figcaption id="garden-photo-caption">
    Hank enjoying himself in the flower garden on a hot day.

That’s helped clarify the role of <figcaption> and its relationship to <figure>. But there’s another important ARIA attribute to include: aria-describedby. The WAI-ARIA spec notes that aria-describedby and aria-labelledby are similar “in that both reference other elements to calculate a text alternative, but a label should be concise, where a description is intended to provide more verbose information.”

It’s not hard to imagine that a full description of an image would naturally appear in the running text of the page where the image has been presented. The purpose of aria-describedby is to make a clear association between the <figure> element containing the image and the image’s description elsewhere. To make that associated, descriptive information discoverable to all users, including those with assistive technologies that do not yet to support aria-describedby, I’ve added to the end of the caption a simple Full description link that points to the descriptive content on the page, using the #-style fragment identifier. (Note that that pattern is what is called for in replacing the old-school longdesc attribute’s function.) When activated, that link will scroll or otherwise point a user’s browser to the unique ID of the element containing the image’s full content. Capable assistive devices can leverage the identifier in aria-describedby to the same effect:

<figure class="photo" aria-labelledby="garden-photo-caption" aria-describedby="garden-photo-description">
  <img src="hank.jpg" alt="Photo of Hank the dog." />
  <figcaption id="garden-photo-caption">
    Hank enjoying himself in the flower garden on a hot day.
    <a href="#garden-photo-description">Full description.</a>
<!-- Elsewhere on the page: -->
<p id="garden-photo-description">
  In the summertime, Hank loves to cool off in the garden’s damp dirt, 
  which always ends up on his tongue... <!-- ...and so on -->

This has become a very strong piece of markup compared to the initial bare-bones <div> example introduced above. And note that it is entirely HTML: no fancy JavaScript or anything is required to make all of this work. It’s an instructive example of the importance of careful treatment and ongoing study of HTML and its evolution beyond the humble, limited alt attribute. I’ve prepared a live example of this, the styling of which I’ll described next.

Foundational Styling for Older Browsers and Newer Semantics

ARIA-enhanced semantic HTML has made the page more accessible on a structural level, including for assistive technologies like screen readers. But because HTML is only a structural language, some CSS, the web’s design language, is required to make those semantic HTML features further accessible to sighted users.

First, not all browsers understand how to style newer elements like <figure>, so the page’s accompanying CSS must include this:

/* CSS */
figure,figcaption,img { display: block; }

That ensures that figure and figcaption display as blocks, separated vertically from other content. Because img defaults to displaying inline with other content, similar to bold or italic text, it also gets added to the list of elements to display as blocks. (If you need foundational selectors for other HTML5 elements, it’s worthwhile to crib from the HTML5 display definitions portion of Normalize.css.)

Unfortunately, unlike past versions of all other browsers, older versions of Internet Explorer will not apply CSS to new HTML elements without the assistance of JavaScript. It’s possible to create the new HTML elements for Internet Explorer using JavaScript’s document.createElement() method (e.g., document.createElement('figure')). However, HTML5 Shiv is a more fully-featured solution for anyone needing to support Internet Explorer versions 9 and earlier. You can include a copy of HTML5 Shiv in your site’s files or load the copy hosted by cdnjs, as in this example:

    Other <head> markup omitted for brevity; <script> tag
    is wrapped in conditional comments that only IE 9 and
    earlier will proceed to process; consult
  <!--[if lt IE 9]>
  <script src=""></script>

I’ve added the following CSS to highlight the descriptive content on the page, using the :target pseudoclass that will be applied when someone activates the Full description link that points to #garden-photo-description. This little enhancement will be especially useful in cases where a layout would not require scrolling, and therefore leave a user puzzled by the presence of an apparently non-functional link in the figure caption:

/* CSS */
#garden-photo-description:target {
  background: rgb(182,219,255);
  transition: background 2s;

The CSS transition property provides a subtle, two-second animation to fade in the background color, potentially giving users time to notice the change on the page as it happens. Again, you can examine the live example showing all of this in action. Its source code is also available on GitHub. The next post will look more closely at the accessibility of images themselves, moving beyond the very basic layout from this example to include responsive design and responsive images.



  1. Hey, Karl,
    Can you suggest some CSS for the #garden-photo-description:target style that could be used universally? In other words is there a way to set up the CSS so that all image full description targets show that background highlight? I’m looking for a shortcut to implement it site-wide without having to add every full description anchor to the CSS file.

    • Absolutely, Traci — You can use the :target pseudo-class all on its own, so that any element whose ID is targeted in the URL will receive the styles you declare. For example,

      :target {
      font-weight: bold;

      would run the text of any element targeted by a URL hash. In the case of a URL like, the page element with id="something" would turn bold. The only caveat here is that if you’re using URL hashes to target anything else, you can run into trouble. If that is a major problem, you could add a class like image-references to all of your HTML related to image references, and then write a CSS selector like this:

      .image-references:target {
      font-weight: bold;

      In that case, only an element whose unique ID is targeted and that has a class of image-references will have its text turn bold.

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